Optical Approach Finds Weak Spots in Jet Thermal Coating

Industrial PhotonicsJan 2018
EDINBURGH, Scotland — A strain-measurement technique uses optical analysis to reveal weak areas in the ceramic thermal barrier coatings that protect jet engine turbines from high temperatures and wear. This technique could be used to predict coating lifetime and could lead to novel thermal barrier coatings that would make engines more efficient.

Researchers from Heriot-Watt University are collaborating with Rolls-Royce to develop and test a way to correlate how strain distribution is related to the coating’s lifetime.

Researchers used a tensile machine to pull a metal specimen with a ceramic thermal barrier coating sprayed on its surface. With a polariscope, they could measure changes in refractive index resulting from this applied strain. Some of the components of the GHz polariscope are seen on either side of the tensile machine. Courtesy of Peter J. Schemmel, Heriot-Watt University.
The lifetime of a thermal barrier coating used on airplane turbine blades can range from as little as 1000 hours up to 10,000 hours at full turbine thrust, even when the coating is applied in the exact same way. Because the lifetime is unpredictable and failure during flight could be catastrophic, turbine blades are scheduled for replacement based on the shortest estimated lifetime.

Researchers from Heriot-Watt University are collaborating with Rolls-Royce to develop and test a way to correlate how strain distribution is related to the coating’s lifetime.

Strain optic coefficients were measured for yttria-stabilized zirconia air plasma sprayed coatings of the same thickness, deposited on substrates of three-millimeter and one-millimeter mild steel. The reflection measurement approach was validated by additional measurements of the stress optic coefficient of bulk yttria-partially stabilized zirconia ceramic, which were in agreement with previously reported transmission measurements.

The technique was tested with pieces of metal sprayed with the same ceramic coatings used on Rolls-Royce turbine blades. Researchers put the pieces into a tensile machine that applied strain by slowly pulling the metal. Changes in the refractive index could be observed when a piece of metal coated with ceramic thermal barrier coating was pulled in a controlled manner. Researchers applied gigahertz (GHz) illumination (280 to 380 GHz) during the process, which traveled through the ceramic coating and bounced off the metal beneath.

Using GHz illumination was key to the technique because GHz wavelengths can travel through some opaque materials, such as ceramics, allowing analysis from within the material.

“With the GHz illumination, we were able to see changes in the refractive index with applied strain,” said researcher Andrew J. Moore. “This shows that our approach could be applied for quality assurance in the future.”

The reflected light was measured using a polariscope to determine how the refractive index of the ceramic changed with the applied strain. Although the team's current optical setup can only acquire point-based measurements, the researchers say the technique could be used with an imaging setup to analyze an entire blade.

“Our strain-measurement technique can analyze the coatings immediately after manufacturing and work to identify the turbine blades that would last the longest in the airplane,” said Moore. “Ultimately, we want to develop an imaging device that would show the strain distribution in the coating of an entire turbine blade, information that would be used to decide if that turbine blade would go into service.”

The researchers recently started experimenting with use of higher-frequency illumination in the terahertz (THz) range, which could improve the technique’s spatial resolution. In collaboration with Cranfield University, they are using their technique to make strain measurements of ceramic-coated metal samples that undergo accelerated aging.

“We will be looking to see when the coatings fail and then correlating that with GHz and THz measurements we took prior to the aging process,” said Moore. “This is a step toward using our technique to identify which coatings fail first.”

The ultimate application of this technique could be to predict the remaining life in thermal barrier coatings. It could also be used to predict the lifetimes of coatings developed to be more reliable or tolerate higher temperatures, which would allow engines to run more efficiently. It might also find use in automotive and nuclear power applications where ceramics are also used as thermal barriers.